1. Field of the Invention
The present invention relates to a secondary battery, and more particularly to a secondary battery having a protrusion formed on a first electrode tab welded to a terminal plate of a cap assembly to increase the contact resistance between the first electrode tab and the terminal plate for easy welding.
2. Description of the Related Art
As portable wireless appliances including video cameras, portable telephones, and portable computers tend to have reduced weight while incorporating more functions, much research has been conducted on secondary batteries which are used as the driving power source thereof. For example, secondary batteries include nickel cadmium batteries, nickel hydrogen batteries, nickel zinc batteries, and lithium secondary batteries. Among them, lithium secondary batteries are widely used in the cutting-edge electronic appliance field because they can be recharged, they can be made in a compact size while having larger capacity, and they have high operating voltage and high energy density per unit weight.
A can-type secondary battery is formed by placing an electrode assembly, composed of first and secondary electrode plates and a separator, into a can together with an electrode and sealing a top opening of the can with a cap assembly.
The electrode assembly is wound while the separator is interposed between the first and second electrode plates. The second electrode plate has a second electrode tab protruding from an upper end of the electrode assembly. The first electrode plate has a first electrode tab protruding from the upper end of the electrode assembly. The first and second electrode tabs are generally of a nickel-based metal.
The cap assembly includes a cap plate, an insulation plate, a terminal plate, and an electrode terminal. After being attached to a separate insulation case, the cap assembly is attached to the top opening of the can and seals the can.
The cap plate is of a metal plate having a size and a shape corresponding to the top opening of the can. The cap plate has a first terminal through-hole formed at the center thereof, into which the electrode terminal is inserted. When the electrode terminal is inserted into the first terminal through-hole, a tubular gasket is attached to the outer surface of the electrode terminal and is inserted together, in order to insulate the electrode terminal and the cap plate from each other. The cap plate an electrolyte injection hole formed on a side thereof. After the cap assembly is attached to the top opening of the can, an electrolyte is injected through the electrolyte injection hole, which is then sealed by a separate sealing means.
The electrode terminal is connected to the first electrode tab of the first electrode plate or to the second electrode tab of the second electrode plate and acts as a negative or positive terminal.
The insulation plate is of an insulation material and is attached to the lower surface of the cap plate. The insulation plate has a second terminal through-hole formed thereon, which is positioned to correspond to the first terminal through-hole of the cap plate 140, so that the electrode terminal can be inserted therein.
The terminal plate is of a nickel-steel alloy (36.5% of Ni and 63.5% of Fe), which is a nickel-based alloy, and is attached to the lower surface of the insulation plate. The terminal plate has a third terminal through-hole formed thereon, which is positioned to correspond to the first terminal through-hole of the cap plate, so that the electrode terminal can be inserted therein. The terminal plate is electrically connected the electrode terminal while being insulated from the cap plate, the electrode terminal being insulated by the gasket tube 146 as it extends through the first terminal through-hole of the cap plate.
The first electrode tab, which is attached to the first electrode plate, is welded to a side of the terminal plate and the second electrode tab, which is attached to the second electrode plate, is welded to the other side thereof. The first and second electrode tabs can be attached by resistance welding or laser welding. However, resistance welding is preferred.
According to a recent method for reducing the internal resistance of secondary batteries, each component is of a metal having a low electrical resistance. In other words, the internal resistance of secondary batteries is reduced to avoid electrical loss within the batteries. Although the terminal plate is typically of a nickel-steel alloy which has a high resistance, it is replaced by a nickel-based metal which has a low resistance to avoid electrical losses caused by the high resistance.
When the terminal plate is of a nickel-based metal, however, the contact resistance between the terminal plate and the first electrode tab decreases and it becomes difficult to weld the first electrode tab. As such, the weldability between the terminal plate and the first electrode tab deteriorates and the welding tends to occur between the first electrode tab and the welding rod.
Such a problem can occur in the case of pouch-type batteries, as well as in the case of can-type secondary batteries.